This invention relates to recovering oil from a subterranean oil shale by means of a conductive heat drive process. More particularly, the invention relates to treating a relatively thick, and relatively impermeable subterranean oil shale by means of a conductive heating process which both creates a permeable zone within a selected portion of the oil shale and subsequently produces shale oil hydrocarbons.
A permeability-aided type of conductive heat drive for producing oil from a subterranean oil shale was invented in Sweden by F. Ljungstrom. That process, which was invented about 40 years ago, was commercially used on a small scale in the 1950s. It is described in Swedish Pat. Nos. 121,737; 123,136; 123,137; 123,138; 125,712 and 126,674. in U.S. Pat. No. 2,732,195, and in journal articles such as: "Underground Shale Oil Pyrolysis According to the Ljungstrom Method", IVA Volume 24 (1953) No. 3, pages 118 to 123, and "Net Energy Recoveries For The In Situ Dielectric Heating of Oil Shale", Oil Shale Symposium Proceedings 11, page 311 to 330 (1978). In the Swedish process, heat injection wells and fluid producing wells were completed within a permeable near-surface oil shale formation so that there was less than a three-meter separation between the boreholes. The heat injection wells were equipped with electrical or other heating elements which were surrounded by a mass of material, such as sand or cement, arranged to transmit heat into the oil shale while preventing any inflowing or outflowing of fluid. In the oil shale for which the Swedish process was designed and tested, the permeability was such that, due to a continuous inflowing of ground water, a continuous pumping-out of water was needed to avoid wasting energy by evaporating that water.
With respect to substantially completely impermeable, relatively deep and relatively thick oil shale deposits, such as those in the Piceance Basin in the United States, the possibility of utilizing a conductive heating process for producing oil was previously considered to be --according to prior teachings and beliefs--economically unfeasible. For example, in the above-identified Oil Shale Symposium, the Ljungstrom process is characterized as a process which ". . . successfully recovered shale oil by embedding tubular electrical heating elements within high-grade shale deposits. This method relied on ordinary thermal diffusion for shale heating, which, of course, requires large temperature gradients. Thus, heating was very non-uniform; months were required to fully retort small room-size blocks of shale. Also, much heat energy was wasted in underheating the shale regions beyond the periphery of the retorting zone and overheating the shale closest to the heat source. The latter problem is especially important in the case of Western shales, since thermal energy in overheated zones, cannot be fully recovered by diffusion due to endothermic reactions which take place about about 600.degree. C."(page 313).
In substantially impermeable types of relatively thick subterranean oil shale formations, the creating and maintaining of a permeable zone through which the pyrolysis products can be flowed has been found to be a severe problem. In U.S. Pat. No. 3,468,376, it is stated (in Cols. 1 and 2) that "There are two mechanisms involved in the transport of heat through the oil shale. Heat is transferred through the solid mass of oil shale by conduction. The heat is also transferred by convection through the solid mass of oil shale. The transfer of heat by conduction is a relatively slow process. The average thermal conductivity and average thermal diffusivity of oil shale are about those of a firebrick. The matrix of solid oil shale has an extremely low permeability much like unglazed porcelain. As a result, the convective transfer of heat is limited to heating by fluid flows obtained in open channels which traverse the oil shale. These flow channels may be natural and artificially induced fractures . . . On heating, a layer of pyrolyzed oil shale builds adjacent the channel. This layer is an inorganic mineral matrix which contains varying degrees of carbon. The layer is an ever-expanding barrier to heat flow from the heating fluid in the channel." The patent is directed to a process for circulating heated oil shale-pyrolyzing fluid through a flow channel while adding abrasive particles to the circulating fluid to erode the layer of pyrolyzed oil shale being formed adjacent to the channel.
Although the thermal conductivity and thermal diffusivity of many subterranean oil shales are, in fact, relatively similar to those of unglazed porcelain and firebrick, U.S. Pat. No. 3,237,689 postulates that "a rapid advance of a heat front" (Col. 3, line 7) can be obtained by exchanging heat between the oil shale and a nuclear reactor cooling fluid and describes systems for using such reactors either located on the earth's surface or in the oil shale deposit.
U.S. Pat. No. 3,284,281 says (at Col. 1, lines 3-21), "The production of oil from oil shale, by heating the shale by various means such as . . . an electrical resistance heater . . . has been attempted with little success . . . Fracturing of the shale oil prior to the application of heat thereto by in situ combustion or other means has been practiced with little success because the shale swells upon heating with consequent partial or complete closure of the fracture". The patent describes a process of sequentially heating (and thus swelling) the oil shale, then injecting fluid to hydraulically fracture the swollen shale, then repeating those steps until a heat-stable fracture has been propagated into a production well.
U.S. Pat. No. 3,455,383 describes the accumulation of partially depleted oil shale fragments within a flow channel such as a horizontal fracture being held open by the pressure of the fluid within the channel. The patent discloses that if the channel roof is lifted to maintain a flow path above such a layer of depleted shale, the overlying formations must be bent and, without precautions, will bend to an extent causing fractures to extend up to the surface of the earth. The patent is directed to a process of intermittently reducing the pressure on the fluid within such a fracture to allow the weight of the overburden to crush and compact the layer of depleted shale.
In a significant portion of substantially impermeable and relatively thick oil shale deposits, such as those in the Piceance Basin, a valuable resource of aluminum is present in the form of dawsonite. In U.S. Pat. No. 3,389,975, directed to recovering aluminum values from retorted oil shales which have been mined out from such deposits, it is pointed out that, in a substantial absence of water, at temperatures of about 1300.degree. F. the dawsonite is converted to crystalline sodium aluminate. Such a water-free retorting can decompose dolomite in the shale to produce carbon dioxide, calcite, and magnesium oxide so that magnesium oxide combines with part of the silicon dioxide in the shale, in a manner permitting a higher recovery of the aluminum values by a leaching process. U.S. Pat. No. 3,502,372, directed to utilizing solution mining to recover dawsonite, indicates that where the pyrolysis is effected by an aqueous fluid, such as steam or the products of underground combustion, it must be conducted at a low temperature and thus relatively slowly, to avoid converting the dawsonite and other soluble aluminum compounds to an insoluble material such as analcite. In U.S. Pat. No. 3,572,838, a similar relatively low temperature pyrolysis is alternated with injections of an aqueous alkaline fluid containing an acid-insoluble chelating agent to aid in leaching dawsonite without forming such insoluble materials.